I you receive compiler warning like:

[DCC Warning] uGC88.pas(24): W1010 Method 'Txt2Db' hides virtual method of base type 'TGC88'

Then just use reintroduce keyword.

Like:

TGC88 = class
procedure Txt2Db;virtual;abstract;
end;

type
TGC88Pdf = class(TGC88)
procedure Txt2Db;reintroduce;
end;